The invention relates to a signal multiplexing system, and more particularly, to a system which multiplexes a composite television video signal and a pulse code modulated time division multiplexed audio signal. The term "composite video signal" is intended to include both a composite color video signal and a composite black-and-white video signal.
The signal multiplexing system of the invention is applicable to the recording and reproduction of signals in a variety of video tape systems and video disc systems. In known video tape system, a composite television video signal and audio signal are recorded on separate tracks, and no signal multiplexing takes place. However, if the multiplexing is possible, the recording capacity can be increased since the audio track can be dispensed with. On the other hand, a variety of video disc systems are known including the TED system which employs a particular record medium of the type disclosed in West Germany Laid-Open patent Specification No. P 20 24 539, the RCA system disclosed in U.S. Pat. No. 3,842,194 and the PHILIPS-MCA system disclosed in U.S. Pat. No. 3,855,426. In these systems, a composite television video signal and audio signal are recorded in a single track on a record medium in a multiplexed form. The composite video signal and audio signal are both frequency modulated while choosing the carrier frequencies so as to avoid an overlap between the occupied frequency bands. However, in these multiplexing systems, the recording level of the audio signal must be chosen considerably lower than that of the video signal, which disadvantageously results in a degraded signal-to-noise ratio of the reproduced voice as compared with that of a television broad casting. The tone quality is also degraded in respects of high and low frequency responses, harmonic content and dynamic range.
In the audio art, it is known to record a plurality of voices in pulse code modulated and time division multiplexed form using a broad band recorder. This recording system is reported in a number of literatures and therefore will not be described in detail. However, briefly, multi-channel analog audio signals are sampled at different timings but with a common period, and the samples are quantized and encoded. The resulting digital signals of all the channels are fed to a parallel-to-serial conversion processor to produce a single digital train. Since the PCM multiplexed signal recording system enables a substantially complete elimination of various distortions, wows or flutters which may occur during the recording or playback process, there is achieved an increased dynamic range over the entire frequency band. In addition, a flat frequency response is achieved over the entire frequency band. However, the occupied frequency band must be increased, which will be considered in more detail below.
In the PCM multiplexed signal recording system, an assembly of bits representing the sampled values is called a "word", and an assembly of words from each channel which follow one after another is called a "frame". Additional bits or characters must be added to such word or frame in order to enable bit or frame synchronization during the transmission, recording or playback of the PCM signal and to reduce or compensate for the influence of code error or drop-out. To this end, a word contains a parity check bit while a frame includes a cyclic redundancy check (CRC) character or frame sync character which corresponds to one word. A PCM signal which is modulated according to the non-return-to-zero (NRZ) system has a frequency band f.sub.p which is given as follows: ##EQU1##
where f.sub.s represents the sampling frequency, W represents the number of bits in a word and F denotes the number of words per frame. The need to contain additional bits and characters for the reasons mentioned above means that the resulting occupied frequency band must be increased in a corresponding manner.